You just have to look at the stats when you pull up the element info. If I remember correctly, you have the light inf in USA Vol brigades, Zouaves, and CSA light infantry. I think the lights have slightly better patrol, screening, and recon values than normal infantry (and possibly higher initiative). Something in between general line infantry and cavalry. In a way that makes them inf +, but not radically different.
The most useful aspect I have noticed is that light infantry tends to be recruited as individual elements, which can be nice in rounding out divisions.